Subsequently, we focus on the experimental signature of vacuum birefringence [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]} arising from the effective non-linear interaction of the electromagnetic fields in Eq. (REF ). See Fig. REF  for the corresponding Feynman diagram. As the signature of vacuum birefringence is expected to scale with a positive power of the energy of the probe photons, the use of gamma photons looks particularly promising.
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